1. Field of the Invention
The present invention relates to a boot installed to a universal joint, for example, a constant velocity joint (CVJ) or the like used in a steering wheel of a motor vehicle or the like, for sealing the universal joint.
2. Description of the Conventional Art
The constant velocity joint in the steering wheel of the motor vehicle is sealed by a boot for preventing leakage of grease in an inner portion and intrusion of muddy water or dust from an outer portion. FIG. 8 is a side view showing this kind of a boot 100 for universal joint by partly cutting it together with a constant velocity joint 200, and FIG. 9 is a cross sectional view showing a state in which the boot 100 for universal joint in FIG. 8 is deformed, together with a part of the constant velocity joint 200.
The boot 100 shown in FIG. 8 is formed by a rubber-like elastic material or the like in accordance with a blow molding, an injection molding or the like, and is constituted by a flexible bellows portion 101 in which a plurality of crest portions 101a, 101c and 101e extending in a circumferential direction and root portions 101b and 101d between them are alternately formed, a large-diameter attaching portion 102 formed at one end of the bellows portion 101, and a small-diameter attaching portion 103 formed at the other end of the bellows portion 101 (refer, for example, to Japanese Unexamined Patent Publication No. 2000-266072, and Japanese Unexamined Patent Publication No. 2003-049944).
The boot 100 is structured such that the large-diameter attaching portion 102 is fastened and fixed to an outer peripheral surface of an end portion of an outer ring 201 of the constant velocity joint 200 formed at one rotating shaft 202 side of the steering wheel by a metal fastening band 104, and the small-diameter attaching portion 103 is fastened and fixed to an outer peripheral surface of the other rotating shaft 203 of the steering wheel rotatably coupled so as to be freely displaced angularly with respect to an axis of the outer ring 201 (the rotating shaft 202) by an internal bearing mechanism of the constant velocity joint 200, by means of another metal fastening band 105.
Further, since the boot 100 is rotated with the outer ring 201 (the rotating shaft 202) and the rotating shaft 203, there is repeated such a deformation that the bellows portion 101 is extended in a half cycle of the rotation and is compressed in the other half cycle, in a state in which the rotating shaft 203 is angularly displaced with respect to the rotating shaft 202. Further, in the case that a joint angle (an angle of the rotating shaft 203 with respect to the rotating shaft 202) is high, the crest portions 101a, 101c and 101e and the root portions 101b and 101d respectively come into contact with each other in the half cycle in which the bellows portion 101 is compressed as shown in the lower half part of FIG. 9. Thus, there is a risk that a contact surface pressure is high, whereby steering torque is enlarged due to increase of rotational resistance and early abrasion is generated.